Everyone: Can do generic adventurer stuff like climbing walls and hiding in shadows. If you're not sure whether they should be able to succeed, have them try to roll under their relevant stat on 1d20 to find out.

Fighters: Get +1 HP per level, and a bonus to all attack rolls equal to their level. Improve their FORT save by two steps. Get +1 to all weapon damage rolls per three levels, rounded down.

Magic-User: Improve their WILL save by two steps. Can cast magic-user spells: one level 1 spell per day at level 1, two per day at level 2, and so on, following the normal B/X progression. Can't cast spells in heavy armour.

Clerics: Improve all their saves by one step. (Someone's watching out for them!) Can cast cleric spells; one level 1 spell per day at level 1, two per day at level 2, and so on, following the same progression as magic-users. (Just as in LotFP, Turn Undead is a spell, not a class ability.)

Elf Fighters: Same as regular fighters, except they improve their REF save instead of their FORT save and don't get the bonus HP per level. Can see in the dark. Heightened senses: can hear and spot things humans usually can't, and only a 1-in-6 chance of being surprised.

Elf Magic-Users: Same as regular magic-users, except they don't get a bonus to their WILL save. Can see in the dark. Heightened senses: can hear and spot things humans usually can't, and only a 1-in-6 chance of being surprised.

Dwarf: Same as regular fighters, except they don't get the damage bonus. They can see in the dark and know loads of stuff about mining and architecture that they can use to spot new constructions, identify unstable areas, bore people to death, etc. They also automatically know the value of any gold or jewels discovered.

Halfling: Same as regular fighters, except they improve their REF save instead of their FORT save and don't get the bonus HP per level. They're sneaky little fucks who are freakishly good at hiding, and have a 5-in-6 chance of surprising their enemies if they don't have a bunch of non-halflings around to cramp their style.

Rules For Other Classes

Kung Fu Monk: Same as regular fighters, except they improve their REF save instead of their FORT save and can't wear heavy armour. They can do 1d6 damage with their bare hands, and while wearing no armour they get +4 AC and can run along walls, jump huge distances, dance like no-one is watching, etc.

The Creature From the Black Lagoon: Like a fighter, but doesn't get the bonus HP per level. Has claws that do 1d6 damage. Can breathe underwater and see in the dark.

Science Guy / Gal: Like a fighter, but they improve their WILL save instead of their FORT save and their attack and damage bonuses only apply when using high-tech weapons, like guns or vibro-axes. (Fortunately, they can make these themselves if given time and materials.) Can do science stuff like identifying chemicals and teaching robots about the true meaning of love. Gains +4 AC when wearing no armour other than a white labcoat.

Big Stompy Robot: Like a fighter, but is treated as wearing plate mail at all times. Inflicts 1d8 damage in melee with its big metal fists, and 2d4 damage at range with its built-in death ray. Always fails REF saves. Incapable of surprising anyone.

Flying Monkey: Same as halflings, except instead of being sneaky they can fly as long as they're only wearing light armour and not carrying anything heavy. They must pass a WILL save to resist obeying orders from anyone who appears to be a witch.

Sexy Green-Skinned Alien: Like a regular Science Guy/Gal, but gains bonus AC from wearing improbably revealing shiny uniforms instead of from wearing lab coats. Gets +1 to all reaction rolls from people attracted to sexy green-skinned aliens. Doesn't get the bonus HP per level.

Mini-Godzilla: Like a Big Stompy Robot, but instead of a built-in death ray, once per day per level it can flame someone in melee combat with its Atomic Breath (3d6 damage). Can only communicate via screaming noises and unconvincing judo chops.

The King of the Bird-Men From Flash Gordon: Like a fighter, except he can fly as long as he's not carrying anything too heavy. Cannot wear any armour except his ceremonial bondage harness (no AC bonus), and cannot use any weapon except his club (1d8 damage). Brian Blessed impressions are obligatory.

Dr Zeus From Planet of the Apes: Like a regular Science Guy, but gains +1 to all reaction rolls from other simians. Lies compulsively about the past.

Intelligence is meaningless in a world of flying monkeys and kung fu robots!

I had been thinking that Int would still be used for the occasional 'skill check'. Honestly, though, the game's minimalist aims would probably be better served by rolling Intelligence and Wisdom into a single stat called 'Mind' or 'Thinkitude' or something. It's not like most of these guys look like they're exactly abounding with wisdom, either.